Variable resistance element



was

Dec. 10, 1935. E. G, LODGE 2,023,603

VARIABLE RES I STANCE ELEMENT Filed March 24, 1931 5 Sheets-Sheet 1 aZM7/E Q I VARIABLE RESISTANCE ELEMENT Filed March 24, 1931 5 Sheets-Sheet 2 Dec. 10, 1935. E. G, LODGE 2,023,603

VARIABLE RESISTANCE ELEMENT Filed March 24, 1931 5 Sheets-Sheet 5 Illlllllllllllllll ||||m|n|||||| W/Zwss ZW Q[,

D843 10, 1935. E, G LQDGE 2,023,603

VARIABLE RES I STANCE ELEMENT Filed March 24, 1931 5 Sheets-Sheet 4 Patented Dec. 10, 1935 UNITED STATES PATENT OFFICE 2,023,603 vaamnm aasrsrrmca ELEMENT of Indiana Application March 24, 1931, Serial No. 524,849

Claims.

The invention relates to a variable resistance element and to a method for manufacturing the same, the resistance element being of a type having wide adaptation to rheostats and potentiom- 5 eters for any one of widely variant uses, and being particularly well suited for use in radio and microphone receiving circuits, for example for controlling the volume of sound reproduction, the element usually including a fine wire wound upon a suitable insulation form and usually cooperating with contacts or terminals of which one or more may be permanently fastened to the coil and one or more may be slidable along the edge of the element to engage successive turns of the wire.

One purpose of the invention is to simplify and cheapen the manufacture of resistance elements for rheostats, potentiometers and the like by coating portions of the individual turns of a made-up 2O coil with low resistance metal, as by exposing the wound coil to an electroplating bath with a selected portion of the outside area, of the wound coil, usually of progressively variant width, shielded from electro-deposition.

A further purpose is to make variation in the position of a movable contact member along its range of movement vary the resistance between the movable contact and a fixed terminal of the coil in accord with any desired predetermined law.

A further purpose is to secure an easy quantity duplication of resistance coils adaptedto vary the resistance between a coil terminal and-a movable contact with the variant position of the movable contact according to any desired law by coating portions of the individual turns included within a suitably variant strip of the made-up coil with a low resistance metal.

A further purpose is to avoid any need for closely gauging the quantity of low resistance coating needed to modify the resistance of a formed coil by making the coating sufficiently heavy to very largely eliminate the resistance of the coated portions of the individual turns as compared to that of the uncoated portions thereof.

A further purpose is to vary the resistances of individual turns of a potentiometer or resistance coil so that the step change of resistance when a movable contact is shifted one turn of the coil shall represent substantially the same fraction of the total resistance between the movable contact and a fixed terminal whatever the position of the contact along its range of movement.

65 A further purpose is to provide a uniformly wound coil with very small resistance steps at predetermined points at which it is known very close adjustments may be desired.

A further purpose is to provide useful and novel forms of potentiometer coils.

A further purpose is to use the same duplicate high resistance coils, each desirably including a great many closely and uniformly spaced turns of very fine resistance wire wound along an insulation form, for the easy manufacture of dupli- 10 cate resistance elements of different ranges of resistance,-ranges of high resistance, ranges of low resistance and ranges of selectively widely variant resistance characteristics with respect to the position of a movable contact member, all 15 including made-up duplicate high resistance coils with suitably variant portions of the individual turns coated with a low resistance metal.

A further purpose is to manufacture duplicate resistance coils comprising an insulation core and 0 many turns of very fine closely spaced high resistance wire, for example a nichrome wire, perhaps 48 gauge, and to use these standard duplicate coils to obtain an easy manufacture of duplicate resistance elements having any one of widely 25 variant resistance, characteristics.

A further purpose is to lower the individual resistance steps from turn to turn of a resistance coil, particularly of a coil of relatively low maximum resistance by providing the coil with an ab- 30 normally great number of turns for a given length of the resistance coil, neutralizing the resistance of anydesired variant portions of the individual turns by coating the said portions with a conducting material, preferably low resistance metal such 35 as copper or silver.

A further purpose is to chromium plate the bearing surface of a movable contact member that slides over the successive turns of a resistance element of the character indicated and to present 40 an uncoated portion of the resistance wires to the contact member.

A further purpose is to lessen or eliminate wear and resultant electric disturbances incident to moving a movable contact element along the 45 upper edge of a resistance element of the character indicated by covering the engaging surfaceswith oil or the like.

A further purpose is to copper plate a resistance wire of nichrome or the like at the portion there- 50 of that is to connect with a fixed terminal in order to adapt the wire to solder to the terminal.

A further purpose is to secure any prescribed very low resistance gradient along a selected portion of a resistor element of the character indi- 55 cated by means of solder joining the successive turns along said portion, usually the low resistance gradient being located at a fixed terminal of the element and extending along the element for a short distance therefrom, and usually predetermined portions of the turns of the element along the range of soldering having been already plated with copper or other low resistance metal well adapted to take solder.

A further purpose is to adapt T pad resistors to easy duplicate manufacture under variant prescribed resistance characteristics by having the contact arms of the resistor elements of the T pad operate as a unit upon standard high resistance coils with the ranges of resistance of the resistor coils relatively adjusted to any desired predetermined relations by suitably plating selected portions of the different coils with low resistance metal.

Further purposes will appear in the specification and in the claims.

I have elected to show a few only of the many different forms of, the invention, showing these to a considerable extent diagrammatically and selecting forms that are practical and efiicient in operation and which well illustrate the principles involved.

Figures 1 and 2 (Sheet 1) are sectional elevations of somewhat different variable resistor structures, Figure 1 including a single resistance element and Figure 2 two resistance elements that may each desirably embody important fea tures of the invention and each figure including mechanism embodying other features of the invention.

ance element and represent the position of a movable contact along the top .of the element and the ordinates represent the electrical resistance between the movable contact and a terminal at the origin end of the element. The figure also includes a fragmentary offset portion of the plot to greatly exaggerated scale, to show the step variations of resistance at successive turnsof the wire with and without the low resistance coating on a portion of the individual turns.

Figure 6 (Sheet 2) is a diagrammatic view corresponding generally to Figure 5 but illustrating avdifierent resistance element in so'far that the turns on the formed coil are coated overprogressively variant portions of their lengths to secure uniformity in the percentage variation of resistance between the spaced terminals at each step, the resistance steps being progressively smaller as the movable contact approaches the stationary contact, the view also including diagrammatic plots of different characteristics of the coil, with the position of the movable contact member along the coil as abscissas, offset portions of some of the plots being shown to greatly enlarged scale.

Figure 7 (Sheet 3) corresponds generally to Figure 6 but illustrates a still difierent resistance element having a movable contact and a fixed contact with turns near the fixed terminal of the coil left uncoated and progressively variant portions of the other turns coated to secure uniformity in the percentage variation of resistance at successive turns of the element as the movable con- 5 tact moves along the portion thereof beyond the uncoated turns.

Figures 8 and 9 (Sheet 4) are diagrammatic views illustrating desirable forms of potentiometer coils, in Figure 8 the coil having fixed ter- 10 minals at the opposite ends of the coil and a movable terminal intermediate the ends and Figure 9 showing a fixed terminal in the middle of the coil and movable terminals intermediate the ends and middle. 5

Figure 10 (Sheet 4) includes two sets of diagrammatic plots, each one of three illustrating the fractional portion of the length of a single turn of resistance wire needing to be coated with low resistance metal to give any desired frac- 20 tional portion of the resistance of the uncoated turn, assuming a given ratio between the specific resistances of the high and low resistance metals and for the several plots individually difierent given ratios between the sectional area of the 25 low resistance coating and that of the high resistance wire; and three other plots respectively illustrating for conditions of the first plots size of errors in the indications of the first plots corresponding to a ten per cent error in the low 30 resistance coating.

Figure 11 (Sheet 4) is a diagrammatic view illustrating one of many different satisfactory arrangements for electroplating selectively variant portions of the individual turns of a made-up coil.

Figure 12 (Sheet 4) is a transverse section upon the line l2-l2 of Figure 11.

Figure 13 (Sheet 1) is a side elevation of a wire-wound strip resistor attached to another form of clamp preparatory to electro-plating unshielded portions of the wire.

Figure 14 (Sheet 1) is an edge elevation of Figure 13.

V Figure 15 (Sheet 1) shows the tapered resistor of Figures 13 and 14 after electroplating and with the clamp removed."

Figure 16 (Sheet 1) is a fragmentary section to enlarged scale taken upon the line It m of Figure 15.

Figure 17 (Sheet 4) is a sectional elevation showing a resistor element suspended in a plating bathby one form of clamp from the cathode bus of the bath.

v Figure 18 (Sheet 4) is an enlarged side elevation of the assembled clamp and resistor coil of Figure 17. i I i t Figure 19 is a side view of the coil of Figure 18 after plating and removing the clamp.

- Figure 20 (Sheet 5) is a longitudinal sectional view illustrating important features of the invention embodied in a T pad resistor, including three resistor element-s operating together, the view being generally similar to Figure 2 except that there is an additional resistor element to G5 adapt it to the T pad! operation, the view also including diagrammatic external electrical connections corresponding to the circuit of Figure 21.

Figure 21 (Sheet 5) is a diagram of a microphone circuit including the T pad resistor of Figure 20.

Figure 22 (Sheet 1) is a view corresponding to Figure 15 modified to illustrate the method of securing soldered connection between a nichrome resistor and an external terminal and also the use of solder or the like to secure a particularly low resistance gradient along any coated portion of the coil.

Like numerals refer to like parts in all figures.

Describing in illustration and not in limitation and referring to the drawings:--

In the structure of Figure 1, 2| represents a contact arm having struck out hook portion 22, and bent back portions 23. Conjugate arm member 24 which is secured to the end of shaft 25 by the screw 26, acts as a fulcrum for arm 2|, slots or sockets 21 being provided to receive finger portions 23.

There is likewise a hook portion 28 struck out from 24, and a coil spring 29 resiliently connects 22 and 28 causing 23 to bear firmly against the conjugate arm 24 at the socket 21. Contact tip 30 is caused to bear uniformly against the edge of the wire-wound resistor element 3|.

Casing 32 is a one-piece metallic stamping, with insulation strip 33 interposed between 3| and said casing. 34 is a disc of insulation fastened by rivets to the housing or casing 32, and supporting the bushing in which the shaft 25 turns. A spring washer W is inserted between the shoulder 25' of shaft 25 and the bushing or bearing 35 to give firm contact.

The wide radial spacing of the casing 32 from the bushing or hearing 35 by reason of its support on the outer portion of the insulation disc 34 and the very considerable spacing between the casing and the grounded metal panel 36 prevent any material capacitance in association with the resistor.

. It is desired that such capacitance be reduced to a small value when the resistor is used directly in a radio frequency path for reasons well known in the art.

3'|.represents insulating washers to insulate the shaft from the metal panel 36, suitable looking nuts 38 being used.

The housing 32 is sealed by means of a metallic disc 39 which is secured in place against the shoulder 50' by the split ring 40 which fits in the groove 50. This provides a casing which is practically air tight, and easily accessible.

In some instances the ring 40 may be omitted and the edge 50 formed inwardly over 39 and lacquer applied to the casing to make a permanent seal.

For best operation it has been discovered that the presence of pure mineral oil on the wire and contact surface materially reduces corrosion, gives better electrical contact, smoother mechanical action, and makes for still quieter operation when varying the volume of reproduction. The illustrated housing is especially adapted to retain a small amount of oil in the bottom, whence the oil is picked up by the end of the arm 2|, each time the arm is turned through the oil which is thereby distributed around the resistor element.

The abutment pin 4| cooperates with the pin 42 to limit rotation of the shaft and contact arm 2|. The pin 4| extends through the flange of the bushing 35 into a keyway in the disc 34 as shown for purposes of assembly.

Another novel detail of the construction consists in the means for firmly and simply holding the arcuate resistor element 3| in engagement with the walls of the casing. This means consists in a toggle element 43 specifically in the form of a bowed clamp and screw 44 (Figures 3 and 4, Sheet 2). J

In place of the screw a rivet may be used or it is possible to simply bend 43 downward relying merely on the set of the metal to hold it. The upturned ends 43' form an acute angle with the core of the resistor element.

The resistor element is inserted in the position shown, the lower edge resting on the insulation disc 34. Next the screw 44 is turned, spreading the ends 43 outward and exerting considerable pressure upon the ends of the resistor element 3|, forcing said element outward against the insulation lining 33 of the walls of casing 32 throughout its length. This firm fit prevents displacement of the resistor element, assists in maintaining the turns of wire in place due to the clamping action between the core of 3| and the lining strip 33, and further allows good heat conduction from the element 3| to the metal casing 32.

Electrical connection may be made between the ends of the resistor element and the terminal members 45 and 45, which are carried by the disc 34, by means of leads which may be a continuation of the wire on the resistor element; in such a case these leads are preferably copper plated to reduce the resistance between the movable contact and the lead to, approximately zero when the contact arm is in zero position'at the lead. For the sake of ruggedness and lowering of resistance, it is preferred to use relatively heavy copper leads for these leads, soldering to the ends of the resistor element.

It has been found that there are no ordinary methods of soldering to "nichrome (nickelchrornium) wire so that when nichrome or the like is selected as a material for the resistor wire, it is preferred to copper plate the end turns of the resistor element, and then solder to the copper plating at 41.

In Figure 2 two of the structures of Figure .1 are combined to operate simultaneously.

A split, inwardly-concave clamp band 43 holds the, bases of the housings together at 53, and an insulation coupling element 5| couples the oppositely directed alined shafts to turn as a unit.

It has been found that in the dual volume controls of a radio circuit. a slight electrical disturbance in one resistor is picked up by the other and amplified in the circuit. In the structure of Figure 2 this disturbance is minimized by the metal plate 39 between the resistors.

This plate is perforated at 39 for the coupling element 5| and theplate acts as a shield for eliminating pick-up disturbances between the resistors.

Plate 39 is in electrical contact with the housing. In this combination construction, it is frequently advisable to ground the metallic housing,

and sometimes it is advisable to do so in the single resistor.

The resistor element 3| included in the struc-.

tures of Figures 1 and 2 may be said to comprise" a standard resistance element suitably modified by coating selective portions of the resistance.

wire of the element with a low resistance metal and one of the more important features of the invention is directed to selectively modifying the.

ample when used in radio receiver sets suitably 40 or even 48 gauge, with the material ofthe resistance wire suitably nichrome.

This basic or standard element, including the insulation core and many turns, of closely spaced resistance wire, lends itself to very easy duplicate manufacture and forms a base for resistor elements of the same standard dimensions, having selectively widely variant predetermined resistance characteristics, for use in widely variant rheostats, potentiometers and the like. a

The selective modifying of the basic element is most suitably accomplished by electroplating the turns of the coil throughout the portions of heir lengths that fall withina selectively variant strip 54 of the element, shielding the wire turns from electroplating except within the strip.

When used as a rheostat. a fixed terminal of the element 3! is suitably fastened to one end of the modified coil and a movable contact member 2| adapted to engage successive turns of the coil at points along one edge of the strip (Figures 4 and 5 to 7).

With a potentiometer there are suitably two fixed terminals, 45 and 45' connected to the end turns of the coil with the movable contact member 2| slidable along one edge of the strip (Figures 3 and 8).

Optionally the potentiometer includes a fixed terminal 55 connecting to an intermediate turn of the coil and two movable contacts 56 and 51 slidable along one edge of the strip (Figure 9).

Let R equal the resistance of one turn of the basic coil, and r the resistance of one turn of the modified coil having a portion p of its length covered with lowresistance metal, forexample copper, then if s is the ratio of the sectional area otthe low resistance coating to the sectional-area .ofthe uncoated wire and the specific conductivity of the coating material is T times that of the material of the basic coil:

(lp)R is thercsistance of theu'ncoatedportion of the turn,

. s tr pe is the conductivity of the coated portionand is the resistance of thecomplete turn. Then In these relations R is definitelydetermined by the material and dimensional characteristics of the basic coil, T is definitely determined by the coating material and s and p can be selected" to give the desired values to r/R which for a given basic coil depend on the desired resistance curve for the completed unit, that is upon varithe strip 54.

' ly the same.

shown in Figure 10 by the sloping straight lines 58, 59 and 80, assuming T=30.

In accord with thoselines it will be seen if,

for example, the low resistance coating covered six tenths of the length of any wire turn, the I resistance of the turn would be reduced very close to 40% of its initial value if s=3, and to about 41% of its initial value if s=0.4

- The curved lines 6|, 62 and 63 Figure 10, indicate respectively for the curves 58, 59 and 60 the 10 percentage of error in the relation between 1) and r/R if there were ten per cent error in determining s. t

It will be seen with p=0.6, a ten per cent error in s would have resulted in less than 1% (about .86 of 1%) error on r/R even with s as low as 0.4 and that with the higher values of s a ten per cent error in determining s would have resulted in much smaller errors in r/R.

The heavier or lighter coatings may either be preferable according to circumstance.

A lighter coating results in a smaller increase in diameter and therefore a lesser reduction in the spacing between the successive turns at the coated portions thereof, and is also cheaper in 26 that it requires less time in the plating bath and uses up less of the plating solution, while the heavier coating permits more easy and more exact duplication of the resistance characteristics of the element and produces an element less affected 30 by any corrosion of the coating.

In both forms when using nichrome and copper respectively for the wire and coating, it is preferable to leave the edge of the element engaged by the movable contact 2| uncoated and 55 also preferable .to chromium plate "the engagingsurface of the movable contact.

In Figure 5 the wireportionswithin a broad strip 54 of substantially uniform width and nearly as long as the element have been coated with copper, each turn along thisbroad strip having the same resistance but a resistance that may be merely 'a small fraction-of that of an-uncoated turn, as of each turn at the portion 64 beyond Loating equal portions ofthe individual turns with copper reduces the resistances ofthe turns equally so'that the resistance. between the movable contact 2! and the fixed terminal'fizwillvarydirectly with the number. of turns intermediate' the contact-and terminal, as represented by the sloping line 65.

The slope of this line is uniform provided that :the strip 54 isof uniform width and that the coating on the diiferent turns is sufficiently near- Theelement is shown as including uncoatecl turns at for some little distance beyond the outer end of the strip 54 so that at-theend of the copper coated strip 54 the rate of increasing resistance changes to that corresponding to the uncoated turns.

The fragmentary plot 66 of Figure 5 is an offset portion of the plot 65 to greatly enlarged scale, at the end of the copper coated portion. 65

It will be seen that the line 65 comprises a succession of steps spaced from one another by the spacing of the successive turns, the height of each step representing the resistance of one turn of the element. 70

As illustrated the steps are perhaps ten times as great along the uncoated portion of the coil, at 64 as along the coated portion 54 while the horizontal spacing between the strips does not vary.

It will be seen that with the rheostat of Figure 5 the individual resistance steps are uniform along the strip 54, each turn between the movable contact 2| and the fixed terminal 45 having the same resistance and the total resistance being therefore equal to the resistance of one turn multiplied by the number of turns.

While in many cases this is desirable in that the resistance between the contact and terminal is then directly proportioned to the deflection of the contact from its zero position, there are other conditions where it has material disadvantages in that the proportionate variation corresponding to the uniform steps then varies inversely with the number of steps making up the resistance between the contact and terminal.

For example, with the resistance steps uniform, a step represents 1% of the resistance between the contact and terminal if the intermediate resistance includes one hundred turns of the coil but when there are only ten turns between the contacts and terminal a step represents 10% of the total resistance between the contact and terminal and when there are only two turn! between the contact and terminal a step represents 50% of the resistance.

In many cases, particularly in radio receiving installations, in the circuits of microphones and the like, closeness of adjustment requires small steps as compared to the total resistance intermediate the terminal and movable contact and with the present invention the turns may be provided with progressively diminishing resistance in order to secure uniformity or near uniformity in the percentage of step variation when the movable contact approaches the fixed terminal.

Thus in Figure 6 at a point 61 spaced a little distance from the zero point for the movable contact 2I 'the intermediate resistance may be that corresponding to say 1.5 turns of the uncoated wire and the portion of coating on the individual turns beyond this point may be progressively diminished from near unity at the point 61 to zero at the outer end of the element. The progressive diminution of the coated portion is optionally selected to make the total resistance between the fixed terminal and the movable terminal follow a logarithmic curve 68 so formed for example that the percentage change of resistance between the contact and terminal will be the same for all the steps beyond the point 61, for example 1% of the intermediate resistance whether the contact is near or far from the point 61.

In the plots of Figure 6, 69 represents the resistance of the coil of Figure 6 for different points of the movable contact member along the coil, 10 the percentage variation of intermediate resistance of each step for the coil of Figure 6, II the resistance of a coil of. the type of Figure 5 over the same range of resistance as that of Figure 6 and T2 the percentage variation of intermediate resistance for the resistor element of curve H.

The fragmentary plots '3 and 13 correspond to oifset portions of the curve 69 to greatly enlarged scale taken respectively at a point 14 toward the zero position of the contact member and at the outer end of the coi1-and illustrate the very small resistance steps toward the zero end of the coil and the much larger steps toward the outer end of the coil.

The plot 15 of Figure 6 corresponds to the plots l3 and 13' except that it represents a correspondingly enlarged portion of curve H at any point along the curve.

It frequently happens that there is enternal resistancein the circuit with the rheostat and that the smallest step should be reckoned with respect to the minimum resistance likely to be in the circuit at the time of adjustment.

In Figure 7 the resistor element comprises an 5 initial portion 11 containing perhaps ninety turns of uncoated wire and a subsequent portion of perhaps five hundred turns along which the turns have variant portions of their lengths coated with copper or other low resistance metal in such a way as to provide optionally uniform percentage of increase at each successive turn of the coil.

With this arrangement an advance or retraction of the contact member by one turn will effect a variation of say one quarter of one per cent of 15 the total resistance included between the contact and terminal whatever the position of the contact member beyond the uncoated portion 11.

Curves 18 and 19 show respectively the resistance of the coil of Figure 7 for different points of 20 the contact member 2| and the resistance of a coil of the form of Figure 5 having the same total range of resistance and the same number of turns as the coil of Figure 7, with uniform resistance steps.

Curves and 8| represent the corresponding variation in percentage of resistance for the two curves l8 and 19 and the fragmentary plots 82 and 83 correspond to enlarged scale offset portions of the curves 18 and 19 at the point 84, the scale being 30 greatly enlarged to illustrate the resistance steps when the contact passes from turn to turn.

It will be seen that the resistance steps of. the curve 82 are uniform and relatively large along the portion 85 having the turns uncoated with 45 copper, that the steps are very small at the beginning of the coated portion and progressively increase to the same value as before the coated portion at the end of the coil.

The steps of the curve 19 are all uniform and 60 of a size intermediate between the large steps at the end of the curve 18 and the small steps at the beginning of the coated turns at the point 84 of the curve.

A particularly desirable resistor element for 45 potentiometer use is that of Figure 8. The resistance steps progressively decrease from near the middle toward the fixed terminals 45 and 45' thereby permitting close adjustment throughout the whole range of the potentiometer.

It will be understood that if. the resistance steps on a potentiometer are uniform throughout the length of the resistor element, with fixed terminals connecting to the end turns of the element and the contact 2| movable along the length of the element between the ends to variantly set the ratio of the resistances from the contact to the respective arms, a fairly close setting may be obtained for ratios near unity corresponding to near mid positions of the movable contact but settings not at all close for points far from unity corresponding to contact positions toward either end of the element.

Thus assume turns between the end terminals, if a desired setting were 1.02, the contact could be set to either 1.00 or 1.04 at turns 50 or 49 respectively with a discrepancy of. only 2% at either position but if the desired setting were 21.5 the contact could be set at turn 5 or at turn 4, the setting in either event being over 11.6% away 70 from the desired setting.

In the potentiometer illustrated in Figure 8, the adjustment may be substantially as close near the ends of the resistor element as in the middle, P1 9 turns toward the end terminals having successively progressively greater portions of' their lengths coated with copper or the like so that the successive turns toward the end terminals are of progressively diminishing resistance.

It sometimes happens that it is known close adjustment will be needed at certain portions of the range and very small resistance steps may be obtained along these predetermined portions of the element, as indicated at 86 and 81, Figure 8 where the portions .of the turns that are covered with copper or low resistance metal have been greatly increased, leaving only very low resistance steps for any desired number of turns at the predetermined points of the range.

It is more usual to connect the fixed terminals of a potentiometer to the end turns of the coil, providing a movable contact to engage the edges of the successive turns intermediate the end turns.

Under some conditions it may be more advantageous to place the fixed contact at an intermediate point and to provide two or more movable contacts along the edge of the elements, as indicated in Figure 9.

The turns in the potentiometer of Figure 9 are shown uncoated for a short intermediate space 88 near the flxed terminal 55, the portion of the element to the right of the fixed contact corresponding generally to the coil shown in Figure 7,

the coating at 90 being optionally such as to provide uniform percentage increase in the resistance between a movable contact 56 on this portion of the element and the fixed terminal 55,

At the other side of the fixed terminal, the resistance is illustrated as varied uniformly at 92 with a succession of spaces 93, where the steps are very small, equal portions of successive turns being coated with copper or the like at 92 and materially greater portions of the turns coated with copper at the spaces 93.

Where the potentiometer is used to obtain a definite ratio between the resistance from one contact 56 to the terminal 55 and the resistance from the terminal to the other contact 51, the actual positions of the movable contacts to\give the definite ratio are variant but mutually dependent in that a selection of one position for one movable contact determines the position for the other contact.

For example the contact 51 may be set at one of the positions 93 having low resistance steps arid the othencontact moved to secure approximate balance after which the first contact is adjusted over the low resistance turns of 93 to secure a very exact balance.

The pattern of the metallic coating strip 54 on sides of the basic or standard coil may widely vary according to the variant desired features of the finished resistor element.

The actual coating may be efiected in any suitable way, as by spraying an exposed portion of a basic coil with finely divided metal, but this coating is believed to be most conveniently effected by electroplating the wire portions included within a predetermined strip 54 of the basic coil while shielding the remainder of the coil from the depositing metal.

The manner of shielding wire portions not within the selected strip or pattern 54 from deposit may be widely varied, and one suitable structure for accomplishing this is illustrated in Figures 11 and 12.

In these figures a basic coil Si is shown suspended in a plating bath 95 Wi h a tank 98.

The made-up coil 3| fits an opening 91 within a metal plate 98 that forms the cathode.

The individual turns of the wire coil engage the opposite inner edges of the plate along opposite edges of the made-up coil, the coil having a small resilient yielding which permits it to be forced easily into the opening but to present each turn at opposite edges of the made-up coil 3i against the cathode plate.

The plate 98 desirably has a thickness slightly less than that of the wound coil and may be used with shields of widely diiferent patterns.

The shields 99 and I00 upon opposite sides of the plate are conveniently alike as to pattern with their inner surfaces provided with a soft lining lili, suitably rubber, adapted to seal the surfaces under the shield plates from the plating liquid. The soft lining iiil while often desirable, may sometimes be omitted in that the. voltage gradient toward electrolytic deposition falls to substantially zero in the space between the plates pressing upon the intermediate resistor.

The outer portions I02 of the shields may be wood or metal and are sufficiently still for clamping the lining tightly against the adjacent face of the coil.

When manufacturing duplicate resistor elements, one shield plate may be left permanently in place, the other removed each time an uncoated coil is to be inserted for plating.

After the coil 3| has been inserted in the opening with the opposite edges of the coil strongly engaging the cathode along opposite sides of the opening, the shield plate that has been removed is put back to an exactly registered position by means of suitable dowel. pins or the like, not shown, and clamped in any suitable way, the rubber lining preventing the liquid of the bath from penetrating the space beneath the shields and thereby effectively preventing any coating 9 below the shield plates.

All portions of the cathode plate that are not covered by the shield are desirably covered with suitable insulation which may be 'paint or the like.

The cathode plate is provided with suitable hook 5 extensions I09 by which it is suspended from the cathode bus bar above the bath.

Anode plates )5 and I06 are shown suspended from anode bus bars I01.

It is usually preferred to use copper for plat- 5o ing the basic resistor wire, particularly if the material of the basic resistor is nichrome, in that copper has low specific resistance and deposits well on the surface of nichrome wire.

In such case the anodes are copper and the electrolyte a solution of copper sulphate dilutely acid with sulphuric acid as will be readily understood. I

Other metals for example cadmium, chromium, nickel or silver may be superimposed on the copgo per coating in order to modify the surface characteristics of the coating or/and to lower the resistance per turn of the coated portion. Any of these metals are usually less subject to corrosion than copper and may therefore, be used to 35 protect a copper coating from possible corrosion.

Other metals than copper, for example cadmium and chromium, readily coat the wire directly and, although not of as low specific resistance as copper, are less subject to corrosion, and '0 may sometimes be preferable to copper for a direct coating. 1

Chromium is exceptionally good for providing wear-resisting characteristics and at the same time will protect any copper coating from corrosion. When extremely fine wire is used, it may be particularly desirable to insure the engagement surfaces of the wire against wear.

Wherever the contact surface of the wire is coated with any easily oxidizable metal such as copper if the resistor is to be used in a radio circuit it should desirably be further coated with a protective covering such as chromium over such parts as are to be engaged by the movable contact, as an oxidation at the points of contact has been found to be productive of noise.

In the finished resistors thus far described, the coated strip has extended for all or most of the length of the element.

In many cases this is neither necessary nor desired, the coil perhaps having a portion I08 I igure 19) near one end -of low resistance per turn and an intermediate taper IIO.

Close duplication of predetermined coatings on different coils is believed to be obtained most easily when shielding the portion of the coil that is not to be coated from the coating solution while exposing the remainder of the coil to the coating action.

Optionally however the resistance taper may be obtained in other ways, as by taperng the thickness instead of the length of coatings on the individual turns, conveniently accomplished by depending the coil from the cathode in such a way that the progressively increasing resistance to the plating current at points of the coil progressively further from the cathode results in a progressive diminution in the rates of deposition.

As indicated in Figures 17 and 18 the uncoated coil 3| is suspended in a plating bath with cathode plate I II making contact engagements at I I2 with the portions of the indiv dual turns at the opposite edges of the coil for a short length toward or at one end of the coil with the individual turns along the remaining portion I I3 of the coil out of contact with the cathode.

As illustrated the opening 91 of the cathode plate III is dimensened for the plate to tightly engage against the opposite edges of the wound coil along the portions H2, and dimensioned to be spaced from the coil edges along the lower portion of the coil at H4 and H5, and below the lower end of the coil to fit the downwardly exlending end of the form 52.

In this arrangement the coil turns along the portion II2 receive coatings that are everywhere substantially uniform except at the po'nts of contact with the cathode plate where the resistor coil remains uncoated.

For the lower portions of the coil progressively more distant from II2 the resistance to the plating current becomes progressively greater, in that these points are progressively further away from the electrical connection to the cathode.

As a result the plating density on the lower port'on II3 of the coil progressively decreases from a maximum at the portion of the coil nearest to the point of engagement at I I2 with the cathode III to a minimum at the lowermost point of the coil. The resistance of individual 'coil turns therefore tapers as illustrated at IIO, Figure 19.

One important feature of the invention includes keeping the individual turns of the resistor coil free from coating at points thereof'presented to the slide contact 2I and in Figure 18 a rubber shield IIG compressed in the recess I I5 engages the coil edge to prevent any deposition at points to be presented to the sliding contact.

In Figures 13 and 14 the coil 3| is suspended by a cathode clamp II I which engages the edge of the coil throughout the length of the clamp and has a taper extension IIO across and engaging the coil.

Here the turn portions covered by the plates 5 of the clamp II! and its taper extension Ill are shielded from deposition by the plates and if the portion of the coil below the clamp is also shieldcd from coating, as by having dipped it in parafiln, the finished coil will correspond to that in- 10 dicated in Figures 15 and 16, the taper II9 being with respect to the length of coating on each turn while at H0 in Figure 19 it is with respect to the thickness of coating on each turn.

A wide and particularly advantageous adapta- 15 tion of the invent'on is in multiple unit resistors having a plurality of resistors adjusted together. Usually the different resistors of a multiple unit have specified mutually dependent ranges, to be predetermined for example experimentally by the 20 difierent customers and yet by the present invention can be made up of units of standard dimensions with standard resistor coils individually modified by the low resistance plating to give the desired ranges and assembled to operate as a 25 single unit.

One multiple unit resistor (Figure 2) has been already described and has a particularly wide application as a multiple volume control for radio receiving circuits.

One of many other applications of multiple resistors having a plurality of resistor coils adjusted simultaneously over mutually dependent predetermined individually difierent ranges of resistance includes the T pad resistor of Figures 85 20 and 21, for use in the circuits of microphone and the like.

These figures show an electric circuit that includes a microphone I20, wire I2I, variable resistance I22, movable contact arms I23 and I24, 40 variable resistance I25, wire I26, transformer primary I21, source of energy I28 and wire I29 back to the microphone, there being also a bypassing connection between the contact arms and the wire I 29, including wire I30, variable resistance I3I and contact arm I32, the three contact arms being in electrical connection.

The transformer primary I21 energizes a transiormer secondary I33 which may feed any suitable amplifying circuit operating a loud speaker. 50

The T pad resistor. Figure 20, includes the three variable resistors I22, I25 and I3I, having contact arms I23, I24 and I32 movable as a unit by angular adjustment of the shaft 25.

I find when the resistor coil is nichrome the contact arms are desirably bronze.

Usually experimental predetermination will have been made by the customer of the best resistance ranges for the three elements I22, I25 and I3I, one such range, for example, being assumed and the others experimentally adjusted thereto to best accommodate the customer's equipment.

As illustrated, movement of the common shaft of the three contact arms in a direction to reduce the resistances I22 and I25 will increase the bypassing resistance I3I, the lay-passing resistance becoming infinity by reason of the contact arm I32 running beyond the coil I3I, before the arms I23 and I24 have reached the positions for zero resistance from the coils I22 and I25.

Figure 20 illustrates three basic units placed in tandem with the arms I23, I24 and I32 all operated by angular movement of the single knob Ill.

The resistor units I36 and I36 are illustrated as being the same as those shown in Figure 2 except that thecoupling I31 between the shafts Ill and I39 is a conductor instead of an insulator.

The added unit I40 and the unit I34 have a common shaft I", the units being securely held together around the hearing or threaded bushing I by the nuts I42 and I48, the nut I43 clamping the fiber plate I44 to the outer face of the nut I42 that in turn clamps the fiber plate 39 to the bushing.

As this construction has already been described in detail for the two-unit volume control of Figure 2 it is deemed unnecessary to further describe the triple unit control, the essential diiference being merely the addition of another' unit and in the specific example the electrical connections needing to be modified so that the contact arms are in electrical connection.

It will be understood that the invention permits the use of standard basic resistors and standard casing units despite widely variant specifications of diiferent customers and thereby is of particularly great advantage in the multiple unit resistors, of which two only have been illustrated, the two-unit volume control of Figure 2 and the three-unit T pad control of Figure 20.

Figure 22 (Sheet 1) illustrates a modified resistor generally similar to that of Figure 15 except that the coated portion of the resistor element has been additionally modified at I45 by coating a short portion of the plated coil with a by-passing film I46 of solder.

It sometimes happens that an extremely low gradient of resistance is desired along a portion of the finished resistor, more usually along a portion of the resistor close to the fixed terminal 4|.

It has been found that the desired low resistance gradient can be obtained by merely coating the coils along the desired range with solder, testing and varying the needed amount of solder for a number of coils, and, after the proper amount has been found that an operator can quickly apply the needed amount with an accuracy suflicient for the intended purpose.

The fixed terminal is desirably soldered to external connections, and one feature of the invention includes the plating of the resistor wire with copper adapting this resistor element to take solder.

It will thus be seen that one of the more important features of the invention includes using standard high resistance coils in combination with easy means for reducing the high standard range to any desired lower ranges, this means including low resistance metal coatings on predetermined portions of the standard coil.

This is a feature of very great commercial importance in that it permits an easy quantity production, not only for example of individual resistors having such variant ranges as different customers may specify but also an easy production of multiple resistors each including a plurality of resistors needing to be adjusted simultaneously and with ranges everywhere along the range of movement of an adjusting knob mutually dependent.

aoaaeos Other features include the special resistor elements adapted to give widely different predetermmed range characteristics; another includes the avoidance of coating on the resistor wire at the points thereof presented to the movable con- 5 tact member which thereby avoids noise producing electric disturbances when the resistor is used as a volume control of a radio receiving circuit, and three of the many other features include the chromium plating of the slidable contact member 10 to further increase the silence and effective life of operation, the copper plating of a nichrome coil wherever it is to take solder and the use of solder for obtaining quickly and easily predetermined abnormally low resistance gradients. 15

It will be evident that the metallic coating on my resistor is effectively a new conductor in shunt with the winding at all points along the winding touched by the coating.

Where I refer to the resistance path portion 20 of the resistor I wish to designate that part of the resistor which is or may be useful in adding resistance to the circuit. excluding the portion, if any which lies dead beyond a fixed terminal, and the portion, if any, useful merely for 25 mechanical attachment of a terminal, and not materially influencing the circuit resistance.

In view of my invention and disclosure variations and modifications to meet individual whim or particular need will doubtless become evident to others skilled in the art, to obtain all or part of the benefits of my invention without copying the structure shown, and I. therefore, claim all such in so far as they fall within the reasonable spirit and scope of my invention.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is:-

l. A wire-wound resistor element comprising a strip of insulation, spaced turns of high resistance wire thereon and a low resistance metallic coating on successively diminishing portions of successive turns along one portion of the element.

2. A variable resistor element comprising an insulation form, and a resistance wire having spaced turns on the form and an electrolytic coating of copper on progressively variant portions of successive turns along a portion of the form.

3. A variable resistor including a nichrome coil,

a movable contact member of bronze and a chromium coating on the contact surface of the contact member.

4. A variable resistor element comprising a strip of insulation, a high resistance wire wound on the strip with the turns thereof spaced substantially uniformly and a metallic coating of low resistant metal on selected portions of the wire, the wire portions along one edge of the said element being free from said coating.

5. A variable resistor element comprising an insulation form, a resistance wire having spaced turns on the form and electrolytic coatings of copper of progressively variant thicknesses on variant portions of successive turns along a portion of the form.

EDMUND GILBERT LODGE. 

